1. Field of the Invention
The present invention is directed to a gradient amplifier for a magnetic resonance tomography apparatus as well as a method for controlling a gradient amplifier of a nuclear magnetic resonance tomography apparatus.
2. Description of the Prior Art
In a magnetic resonance tomography apparatus, a magnetic field gradient is generated by a gradient coil which is in turn connected to a gradient amplifier. The gradient amplifier supplies voltages on the order of magnitude of a few hundred through a few thousand volts in order to produce an exactly regulated current in the gradient coil. This current fluctuates in a predetermined current curve, for example, between 0 and 300 A.
The current flowing through the gradient coil must be extremely precisely regulated. Deviations of only a few mA can already produce imaging disturbances.
German OS 40 24 160 (corresponding to U.S. Pat. No. 5,111,378) discloses a gradient amplifier as well as a method wherein a reference value and an actual value of the coil current are supplied to a control unit which generates a setting value dependent on the relationship between the reference value and the actual value, the control unit supplying the setting value to a modulator which produces at least one output stage drive signal therefrom, the modulator supplying the drive signal to an output stage to which the load is connected. Due to the high precision demands mentioned above, all components in this amplifier are designed for an extremely high stability and resolution in order to generate signals with a precision in the ppm range. This requires expensive signal components for the entire amplifier system and a complicated balancing in the manufacture as well as during every maintenance event.
German OS 197 06 756 discloses a gradient amplifier for a magnetic resonance tomography apparatus with a control means having a control input for a digital control word. A current transfer function of the control means can be selected from a number of predetermined transfer functions with the control word. As a result, the gradient amplifier can be simply set to different control characteristics without having the readjustment require a manual balancing.